Tiffany is packing her bags for her vacation. She has $7$ unique books, but only $5$ fit in her bag. How many different groups of $5$ books can she take?
Solution: Tiffany has $5$ spaces for her books, so let's fill them one by one. At first, Tiffany has $7$ choices for what to put in the first space. For the second space, she only has $6$ books left, so there are only $6$ choices of what to put in the second space. So far, it seems like there are $7 \cdot 6 = 42$ different unique choices Tiffany could have made to fill the first two spaces in her bag. But that's not quite right. Why? Because if she picked book number 3, then book number 1, that's the same situation as picking number 1 and then number 3. They both end up in the same bag. So, if Tiffany keeps filling the spaces in her bag, making $7\cdot6\cdot5\cdot4\cdot3 = \dfrac{7!}{(7-5)!} = 2520$ decisions altogether, we've overcounted a bunch of groups. How much have we overcounted? Well, for every group of $5$ , we've counted them as if the order we chose them in matters, when really it doesn't. So, the number of times we've overcounted each group is the number of ways to order $5$ things. There are $5! = 120$ ways of ordering $5$ things, so we've counted each group of $5$ books $120$ times. So, we have to divide the number of ways we could have filled the bag in order by number of times we've overcounted our groups. $ \dfrac{7!}{(7 - 5)!} \cdot \dfrac{1}{5!}$ is the number of groups of books Tiffany can bring. Another way to write this is $ \binom{7}{5} $, or $7$ choose $5$, which is $21$.